1. Field of the Invention
This invention relates to a rotatable cylindrical magnetron sputtering apparatus and a related process. More specifically, the invention relates to a cylindrical target assembly for a cylindrical magnetron sputtering device which includes a “floating” target portion where the target portion is metal, metal oxide or ceramic and is not bonded to any backing tube, and which cylindrical target assembly allows the target portion to heat up uniformly and expand, thereby allowing the cylindrical magnetron to operate at increased power levels.
2. Description of Related Art
A typical magnetron sputtering device includes a vacuum chamber having an electrode contained therein, wherein the electrode includes a cathode portion, an anode portion and a target. The term electrode is oftentimes referred to in the industry as a cathode. In operation, a vacuum is drawn in the vacuum chamber followed by the introduction of a process gas into the chamber. Electrical power supplied to the electrode produces an electronic discharge, which ionizes the process gas and produces charged gaseous ions from the atoms of the process gas. The ions are accelerated and retained within a magnetic field formed over the target, and are propelled toward the surface of the target which is composed of the material sought to be deposited on a substrate. Upon striking the target, the ions dislodge target atoms from the target, which are then deposited upon the substrate. By varying the composition of the target and/or the process gas, a wide variety of substances can be deposited on various substrates. The result is the formation of an ultra-pure thin film deposition of target material on the substrate.
Over the last decade, the cylindrical magnetron has emerged as the leading technology for sputter coating on both rigid and flexible substrates. The rotating cylindrical target surface provides for a constant sputtering surface, thus eliminating the traditional erosion groove and large non-sputtered areas associated with planar targets. Further, the cylindrical target eliminates large areas of dielectric buildup that can lead to arcing, material flaking, debris and other process instabilities.
Transparent conductive coatings are used in a broad variety of devices, which may include Plasma, TFT or LCD televisions, and computer monitors to solar cells, to resistive heating windshields for aircraft and trains, with a broad variety of applications in between. Typically, these transparent conductive coatings are made of indium tin oxide (ITO) or aluminum zinc oxide (AZO) thin films. Further, the majority of these devices have the transparent conductive coatings applied by sputter deposition.
One method of applying ITO and AZO films, in order to obtain the best electrical conductivity and visual transparency, is to apply the coatings to substrates at elevated temperatures, specifically, above 200° C. The problem is that the films can be optically reflective in the infrared spectrum and, therefore, as the film grows in thickness it becomes necessary to continually apply more heat in order to maintain this temperature for the duration of the coating cycle.
Some problems associated with sputtering these ITO or AZO targets are target cracking and the formation of nodules. When a target cracks, there is an increased opportunity for the generation of particulate contamination, as well as arcing on the surface of the target, thereby causing production yield loss. Nodules are known to be sites of impurities which form on the target surface and which may involve inhibiting the quality of the film formation, as well as slowing down the deposition rate, thereby causing yield loss and throughput.
FIG. 1 shows a prior art rotatable cylindrical magnetron sputtering device 8 that includes a cylindrical target assembly 10 and a drive assembly 12 rotatably connected to the cylindrical target assembly 10, which is similar to that disclosed in United States Patent Application Publication US 2008/0012460 A1 (hereinafter “the '460 publication”), published on Jan. 17, 2008, and which is hereby incorporated herein by reference in its entirety. The drive assembly 12 includes a motor 14 coupled to a drive shaft 16, wherein the drive shaft 16 is rotatably connected to the cylindrical target assembly 10. The structure and operation of the drive assembly 12 including the remaining components for the rotatable cylindrical magnetron sputtering device 8 is anticipated to function similarly to that disclosed in the '460 publication.
Referring to FIGS. 1 and 1A, the cylindrical target assembly 10 includes a hollow cylindrical target 18 having an inner surface 20A and an outer surface 20B, a backing tube 22 bonded to the inner surface 20A of the target 18. The assembly 10 further includes a central member 24 such as a shaft or sleeve received within the cylindrical target 18, a cathode body 26 having a base plate 28 attached thereto, wherein the base plate 28 is attached to the central member 24 for supporting the cathode body 26. Attachment of the base plate 28 to the central member 24 may be accomplished using a clamp or any other suitable clamping arrangement known in the art. A magnet receiving chamber 30 is defined in the cathode body 26, wherein a magnet arrangement 32 is received within the magnet receiving chamber 30. The cylindrical target 18 and the attached backing tube 22 is held in place by an annular target retaining assembly 34 (such as a clamp arrangement) on each end thereof, which is in communication with the drive assembly 12, such that the drive assembly 12 is adapted to rotate the cylindrical target assembly 10 around the cathode body 26 and to introduce high current power into the drive shaft 16 when the shaft 16 is rotating. The retainer assembly 34 may include an end cap 35 rotatably connected to a clamping member 36, wherein the clamping member 36 has a step portion 37 defining a recess 37A therein for receiving the target 18. The retainer assembly 34 may also include any other suitable clamping arrangement known in the art.
In order to promote long target life without target cracking, it is the present practice to mechanically bond the cylindrical target 18 to the backing tube 22 as shown in FIG. 1. The backing tube 22 is typically made of copper with the goal of removing a sufficient amount of heat during the sputtering process. The bonding agent used is typically indium or an indium alloy. This practice is both costly and time consuming. In order to reduce the formation of nodules, it is known to attempt to sputter etch the largest area of the target surface as possible so that the nodules are cleaned or etched away before they have a chance to form. Also, to maintain the production rate, it is necessary to stock an extra supply of bonded targets on extra backing tubes, which can be used when the first set is sent out to the vendors for removal and reclamation of the unused ITO and for re-bonding of a new target.
There is, therefore, a need in the art to develop a device and process which allows the target to heat up uniformly and to sputter at powers and rates much higher than traditionally accepted, which eliminates the formation of nodules and the need to bond the target to a backing tube, and which results in a resistance to the cracking of the target.
It is, therefore, desirable to provide a cylindrical magnetron sputtering device which includes a cylindrical target assembly which may be adapted to decrease the amount of heat transferred to the target, while still allowing the magnetic fields to travel to the target for the sputtering of the target against a substrate.
It is further desirable to provide a cylindrical target assembly wherein the target is not bonded to the backing tube, nor does it have direct mechanical contact with the backing tube such that the target may be changed quickly resulting in increased production rates, and the same backing tube may be used for several replaced targets.